Generation of very short microwave pulses



C. D. HARDIN ETAL V GENERATION OF VERY SHORT MICROWAVE PULSES Filed NOV. 5, 1957 FILAMENT VOLTAGE l6 MAGNETRON.

L cmswu- DETECTOR VIDEO MODULATOR ASSOCIATED RADAR IFIER OIRCUITRY AMPL R F ENVELOPE OF INVENTORS -BLOCK|NG OSCiLLATOR OUTPUT PULSE MAGNETRON OUTPUT PULSE x 2O MICROSECONDS PER DIVISION HM/A W25 W2. Ma y mvm 2,90s,s70 V GENERATIONOFLVERYSHORT MicRowAvE PULSES Clyde Hardin, Rockville, Md., and James Salerno, Washington, D.C.,- as'signors to the'United States of America as represented by the Secretary of the Army ApplicationNovemher s, 1957, Serial No.694,6s7 v 1 Claim. (on. 33147 (Granted under Title ss,U.s. Code 1952 see. 266) resolution that-can be obtained in a radar system. In the prior art, a considerable reduction in pulse width is obtainable using conventional pulse forming thyratron methods, but such methodsare complex, bulky, and are not able to adequately provide microwave pulses in the 10 millimicrosecond range as is desired in a high resolution system. A further difiiculty of prior art methods arises because of their inability to provide very short pulses havvnied Patent 2,908,870 Patented Oct. 13, 1.959

will reveal the complex modulators that were employed in these magnetron radar systems. The use of the ferrite transformer blocking oscillator with a magnetron generator not only provides the desired short pulse and high tion rate pulses in accordance with the invention.

Figure 2 is a graph showing the blocking oscillator modulating pulse and the resulting magnetron microwave pulse fora particular embodiment of Figure 1.

Figure 3 is a block diagram of a short pulse radar system incorporating the means of Figure 1.

ing a high repetition rate. Thus, high power sources are necessary toobtain adequate average signal strength.

It is an object of this invention, therefore, to provide improved means for generating very short microwave pulses at a high pulse repetition rate.

It is a further object to provide compact and simple l0 millimicrosecond range. a Y

An additional object of this inventionis'to provide a high resolution short pulseradar' system having ahigh pulse repetition rate. i

In a typical embodiment of the invention, these'objects are accomplished by means of a blocking oscillator modulator which utilizes a ferrite core transformer. The blocking oscillator is connected to a microwave generator'so that the blocking oscillator pulses modulate the microwave output of the generator. The high permeability of the ferrite allows the use of a small number of coil turns in the blocking oscillator transformer so that the capacity is held to a minimum and very short pulses at 'a low impedance output can be produced. Since the blocking oscillator circuit only consumes power during the time it is pulsing, the average-power required will be small, making possible 've'ryshort high power pulses having a high repetition rate. Although blocking oscillators, ferrites, and microwave generators have been known in. the

In Figure l, a blocking oscillator modulator unit 10 pulse modulates a magnetron 12 to provide microwave output pulses. The modulator unit 10 comprises a blocking oscillator circuit employing a parallel-connected twin triode 25 and a ferrite core transformer 35 connected for blocking oscillator operation. The blocking oscillator circuit is shown as self-oscillating but may be trigger-operated if so desired. The twin-triode 25 has parallel connected plates. 26,-grids 27, groundedcathodes 28, and a common filament 30 which has one side grounded and the other side connected to a filament voltage source. A capacitor 110 serves as a conventional filament capacitor. The ferrite core transformer 35 has bifilar wound plate and grid windings 36 and 37 connected to provide the feedback necessary for blocking oscillator operation, and bifilar wound output windings 33 and 34 connected in parallel in the cathode leads'of the magnetron 12 to provide modulating pulses to the cathode 50. These output 1 windings 33 and 34 also provide paths for the flow of filameans for generating high power microwave pulses in the ment current to the magnetron cathode 50 which is of the common cathode type. The black dots adjacenteither the top or bottom of the transformer windings 33', 34, 36, 37 are intended to indicate the polarity connections of these windings in the conventional manner. The plate winding art for some time, no one has combined these elements 37 has one end connected to B+ in the conventional man,-v nerand a capacitor 56 serves as the B+ capacitor.

The blocking oscillator circuit is adapted so that the pulse width obtained at the output windings 33 and 34 is determined by the inductance-capacitance characteristics of the transformer 35. Because of the high permeability of the ferrite core employed, onlya small number of coil turns is necessary to provide the inductance required for the windings. It is possible, therefore, to greatly reduce theassociated transformer capacitance which .acts to stretchthe pulse width. Thus, using stan-v dard techniques for reducing other strap capacitances, it is possible to obtain very short pulses at 'the output windings33 and 34. In addition, because of the small number of coil turns neces'sary,zvery close coupling between windings can' be obtained making possible the production of high voltage pulses at avery low'ioutput impedance; Even closer coupling and a 'smaller-capacitance can be obtained bybifilar' winding the plate and grid windings 37 and 36, and bifilar winding the-output these windings in the transformer 35 are intended to represent these bifilar windings.

The time constant of the circuit 101 comprising a resistor 102, a potentiometer 105 and a capacitor 104 determines the repetition rate of the blocking oscillator pulses in the conventional manner. The repetition rate may be varied by adjusting the potentiometer 105. The capacitor 104 is chosen so that the blocking oscillator pulse is determined by the inductance-capacitance characteristics of the transformer 35.

Actual design of the blocking oscillator circuit is dic fated by the pulse Width and repetition rate desired, and the characteristics of the microwave generator to which the output pulses are applied. Such characteristics of the microwave generator are: voltage level at which oscillations start, and input impedance. Selection of tube type is determined not only by figure of merit (G /21rC), but also by plate dissipation and voltage breakdown consider ations. Because the blocking oscillator circuit only consurnes power during the time it is pulsing, the average power required will be small, making possible the production of high power pulses at a high repetition rate without the need for a tube 25 capable of handling high average powers.

As described previously, the use of a ferrite core in the blocking oscillator transformer 25 makes possible a close coupling and aredu'ction in capacitance toan extent where very short high voltage pulses at a low impedance output are possible. In general, it may be said that the material of highest permeabilitygives the optimum performance, subject to limitations of saturation and temper'ature. Loss factor is not so important since the blocking oscillator transformer 35 must be heavily damped; loss factor only comes into consideration when the a the voltage of maximum magnetron output.)

2 is a graph showing the blocking oscillator output pulse, and the resulting microwave pulse obtained from the magnetron for this specific embodiment of Figure 1. particular embodiment has been packaged into a volume of less than 50 cubic inches.

The L-3028A is a rugged magnetron tube developed for military use. It has an efficiency of produces a peak power of 150 watts with 800 volts applied, and is capable of delivering 2 watts of average power. Although the manufacturer of the L-3028A quotes requirements for a 50 millimicrosecond rise time in the applied pulse, we have been able to obtain 10 millimicrosecond microwave pulses without difficulty using the circuit of Figure 1. It is to be expected that other magnetrons of the L-3028A type are also capable of producing very short pulses of this order of magnitude in the circuit of Figure 1.

One of the problems encountered with magnetrons is that the rise time of the applied pulse must not be too short in the region ,where the pi mode begins to form. (The pi mode begins to form at voltages somewhatunder It has been found that too short arise time in this region results in the production of undesirable modes and sparking of the cathode. This is probably one of the reasons for the restriction in rise time quoted by the manufacturer of the L'-3 028A. The use of a blocking oscillator type pulse overcomes this problem because it is essentially a'dam'ped sine wave and has the necessary characteristic slope reductiorl as it nears its peak value to permit the proper formation of the pi mode. .It is possible, therefore, by

s using the blocking oscillator pulses produced by the cirpower dissipation i-s sufiicient to raise the core temperamm toward its curie point. Since materials of higher permeability saturate easily and have low curie temperature's, the choice of material must be based on the operation of the particular circuit involved. The ferrites found to be most useful are Ferramic C, E, and G, Ferra'mic C being especially useful for high temperature operation because of its high curie temperature. The windings of the transformer are adapted to match the input impedance of the'inicrowave generator and provide the necessary oscillation voltage. Because of the close coupling obtainable as described previously, very short high voltage pulses can b'e'produc'ed having an output inpedance which is low enough to match the input impedance of the microwave generator. In Figure 1, two output windings 33 and 34 are provided to give the necessary voltage and impedance match to the magnetron 12.

In a specific embodiment of the invention, a 5687 twin triode is used for the tube 25 and a Litton Industries L30 28A'magnetron is used for the magnetron 12. The transformer core is a 1% inch toroid'o'f Ferramic G, the bifilar wound grid and plate windings 36 and 37 have 8 andS turns respectively, and the bifilar wound output windings 33 and 34 each have 13 turns. B+ is 500 volts, thefila'rn'ent voltage is 6.3 volts A-C, and the filament and B+ capacitors 110 and 56 are .01 and .047 microfarad respectively. In the circuit 101, the resistor 102 is 20,000 ohms, the potentiometer 105 is 100,000 ohms and the capacitor 104 is 60 micromicrofa'rads. For these values in the circuit of Figure. 1, the blocking oscillator output windings 33 and 34 feeding the magnetron cathode 50 cuit of Figure l, to obtain very short microwave pulses from magnetrons of the L-3028A type.

Having provided means for generating very short high repetition rate microwave pulses, it is now possible to design a high resolution short pulse radar system. For any particular system, the antenna, receiver, and intelligence circuitry, can be chosen to meet the requirements of the specific use. Figure 3 is a block diagram showing one system in which the circuit of Figure 1 may be incorporated. A transmitting antenna 16 is connected .to

1 associated radar circuitry 23 for utilizing the modulator and received pulses to provide adesired indication of the target. In a particular system of Figure 3, in which is incorporated the particular embodiment of Figure 1, described previously, the video amplifier 22 is a distributed amplifier designed to have a bandwidthof about 50.n1egacycles and a gain of about 90 db. A decoupling of 75 db between the transmitting and receiving antennas 16 and 17 is provided to avoid overdriving by the leakage pulse. Using sucha system, a high resolution radar system was obtained which operated out to as far as 100 feet, and could detect changes in distance as small as 6 inches.

It should be noted that the invention described may be extended to higher power systems and a wide variety of applications where a high resolution radar system is desired. Although a crystal video system is illustrated in 3 Figure 3, the invention may also be incorporated into a provide approximately an 800 volt pulse at an output superhetrodyne system which uses an I.-F. amplifier. Also, where high decoupling between transmitting and receiving antennas is difficult to obtain, suitable gating may be employed to avoid overdriving by the leakage pulse. Some of the possible applications for which the invention may be employed are altimeter, surveillance radar, radar mapping and radar warning systems.

It will be apparent that the embodiments shown are only. exemplary and that van'ous modifications can be i made in construction and arrangement within the scope of the invention as defined in the appended claim. We claim as our invention: p Means for'generatingvery short microwave pulses at and a blocking oscillator circuit adapted to supply said' modulating pulses, said blocking oscillator circuit comprising a vacuum tube and a transformer connected for blocking oscillator operation, said vacuum tube having at least plate, grid and cathode elements, and said transformer having a ferrite core and a plurality of windings wound thereon, said plurality of windings including bifilar wound plate and grid windings, and two bifilar wound output windings, said output windings being connected to said cathode to provide said negative modulating pulses, said ferrite core having the highest possible permeability subject to limitations of saturation and temperature.

References Cited in the file of this patent UNITED STATES PATENTS 2,570,295 Vantine, Jr. Oct. 9, 1951 2,611,089 Valeton Sept. 16, 1952 2,694,149 Gross Nov. 9, 1954 OTHER REFERENCES Gated Marker Generators, by G. D. Jensen. Electronics, April 1954, page 177. 

